All U.S. patents noted below are fully incorporated herein by reference.
Polyurethane foam articles that include no coloring agents therein are, at the production stage, white in appearance. Such an uncolored article, particularly in slabstock form, is highly desirable for many different potential end-uses, ranging from mattresses, to novelty items and toys, to apparel accessories (i.e., women's shoulder pads), to undergarments, and the like. However, it has long been a problem that such white polyurethane foam articles (such as slabstock, rigid, or other types) exhibit a very high propensity for deleterious discolorations and yellowing due to a number of factors. Ultraviolet exposure, or lightfastness problems, reaction with oxidative atmospheric chemicals, thermal degradation or scorching during exothermic production all appear to contribute to such discoloration problems. As such, the ability to provide long-term white colorations has been a struggle for the polyurethane foam industry.
As noted above, such yellowing and/or discoloration problems appear to be the result of a combination of factors. Light stability, including ultraviolet exposure, appears to have a significant effect on products based on isocyanates, particularly upon aromatic isocyanates (one of the primary reactants to form the vast majority of polyurethane foams). Yellowing is a natural result thereof upon sufficient exposure to light and there is little protection from such a deleterious result without a protective additive or selection of more resilient polymer for polyurethane production.
Also contributing to such discoloration issues is gas fade, otherwise known as the exposure of such polyurethane foams to combustion byproducts that exist and are pervasive within many environments. The highly oxidative species generally present within such atmospheric materials (such as, for example, nitrous oxide) appear to readily react with reactive foam constituents such that modification of color therein readily occurs as a result. Furthermore, thermal conditions during the highly exothermic reaction of low density urethane foam formulations contribute as well to potential discoloration, particularly within the center of the target article (since this is the location of the greater exothermic activity during production). Brittleness of the foam, as well as yellowing and even browning are distinct and strong possibilities as a result. All together, the ability to produce white polyurethane is just as difficult as retaining white colorations within produced articles due to these highly problematic and readily pervasive conditions.
Since removal of such conditions is, for all intents and purposes, impossible, additives have been developed to remedy such problems individually. Benzotriazole-based additives have been found to alleviate a certain and significant level of discolorations resulting from ultraviolet light exposure, for example. White coloring agents (such as titanium dioxide, for example) can also be incorporated to mask potential yellowing, but such a solution has marginal benefit and can adversely affect the physical properties of the foam. As a result it is considered unsatisfactory the majority of the time. At least for this individual ultraviolet and/or light exposure issue, the aforementioned benzotriazoles appear to provide a certain degree of reliable protection.
Thermal exposure also appears to contribute problematic discoloration properties to such articles. Basically, it is well known that such polyurethane foam products require the presence of at least one catalyst to effectuate the desired reaction between the necessary polyol and isocyanate components. The most prevalent catalysts, due to cost in producing, using, and disposing, are tertiary amine-based compounds. These catalysts include hydroxyl terminated types, such as the most popular types used throughout the industry, DMEA (dimethyl ethanol amine), DABCO TL catalysts (blends of triethylene diamine and 2-[[2-(dimethylamino)ethyl]methylamino]ethanol), and Texacat ZF10 (N,N,N′-trimethyl-N′-hydroxyethyl-bis(aminoethyl)ether). These catalysts unfortunately present the ability to exaggerate certain problems within the resultant foams, most notably scorch discoloration and/or degradation. Scorching is a common occurrence within exothermic foam-producing reactions, particularly when air flow is minimized within the foam-making procedure.
Apparently, such catalysts react readily with free isocyanate due to their reactive hydroxyls within the polyurethane and/or colorants and/or other additives present. In particular, such heat generation is pronounced due to the avoidance of CFC-type blowing agents (which dissipate heat during high temperature exothermic reactions upon use). As it is, the foam blowing agents now utilized throughout the industry are ineffective at dissipating the very high temperatures generated during the curing process. These high temperatures appear to oxidize the polyol due to the reaction with free radicals and hydroperoxides generated during the curing process. Such compounds react readily with hard polyurethane segments within the foam product to form quinonoid-type structures that consequently cause color bodies to form. These resultant color bodies thus create discolorations within the final foam product since they are of a different color than the desired foam product. Apparently, such high temperature discolorations and degradations more readily occur between about 30 and 60 minutes after foam generation (during gelation and blowing of the foam-producing composition) has taken place. During such exothermic oxidation reactions, the foam is then “burned” by the high temperatures thereby producing the highly undesirable discolored areas within the resultant foam article. Such scorching may also cause degradation of “burned” portions of foam to the extent that the affected areas exhibit much different physical properties than the unaffected foam. In such an instance, generally the scorched portions will become more brittle (and more prone to tearing or a loss in resilency) than the properly formed foam.
Attempts at alleviating these particular problems have included the addition of potentially environmentally unfriendly, and potentially toxic antioxidants, such as 2,6-di-t-butyl-4-methylphenol (BHT), octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propanoate (Irganox® 1076, from Ciba Geigy), and octyl-3(3,5-di-t-butyl-4-hydroxyphenyl)propanoate (Irganox® 1135), within the curing process. This has proven only marginally effective; however, again due to the expense and the large amount of such antioxidant compounds required, as well as the large amount remaining within the foam (which may be troublesome due to environmental and safety concerns), such a procedure is necessarily avoided if at all possible. Since there is a 15 to 30 minute window of opportunity to control high temperature exposures, some foam producers have practiced forced air cooling of the foam-producing composition in the past to reduce scorch problems. Unfortunately, however, the cost involved with providing the necessary degree of heavy air flow (particularly in a specific limited direction) is prohibitive. With both procedures, the costs involved have resulted in transferred costs to the foam purchaser and end user. Alternative methods, either simpler and less expensive in nature, have not been forthcoming within the industry.
Thermal discoloration problems are not easily cured with the aforementioned benzotriazole compounds as foam additives, if at all, either. In greater detail, and as suggestions for remedying scorch problems in such polyurethane foams, U.S. Pat. No. 6,541,531 teaches certain organic cyclic esters as additives for such purpose. Also, the aforementioned BHT and similar derivatives thereof provide a certain degree of thermal protection to such foam articles.
However, such antioxidant compounds (as noted above, including BHT, etc.) for anti-scorch and benzotriazoles for UV/light protection cannot simultaneously protect the target polyurethane foams from highly oxidative combustion byproducts present within many atmospheric environments, particularly in urban and less populated centers. Such a phenomenon as gas fade has been largely ignored within the polyurethane industry due to the difficulties of protecting such foam articles from the oxidative species so pervasive around the world. As it is, it appears that the additives utilized for such protective reasons actually react readily with such combustion byproducts that discoloration, although alleviated during light exposure or scorching possibilities, is persistent, if not worse, due to gas fade exposure. The above-noted BHT is particularly susceptible to nitrous oxide reaction such that a pH dependent color body is generated that severely discolors the article. Such a colored product generates a highly undesirable unnatural aged appearance within the resultant article. Though BHT provides benefit in light and thermal testing, it thus exhibits severe limitations and deleterious effects in response to gas fade.
As such, it is apparent that this combination of factors has not been properly considered together, nor alleviated, at least through simple additive methods and formulations. Thus, there currently exists no effective remedy to such a three-pronged problem for producing and retaining naturally colored polyurethane foams with white appearances. The industry demands such a currently nonexistent improvement.